see what happens when you add a materialized view... this will be much quicker..

create VIEW vx_agg WITH SCHEMABINDING

as

SELECT --( COUNT_big(*) * SUM(col1*col2) - ( SUM(col1) * SUM(col2)))/( COUNT_big(*)* SUM(col1*col1)-( SUM(col1)* SUM(col1))) AGG

COUNT_big(*) cnt

,SUM(col1*col2) col1col2

,SUM(col1) col1

,SUM(col1*col1) col1sq

,SUM(col2) col2

FROM dbo.test_table

GO

CREATE UNIQUE clustered INDEX IX_AGG ON vx_agg (cnt)

SELECT ( COUNT(*) * SUM(col1*col2) - ( SUM(col1) * SUM(col2)))/( COUNT(*)* SUM(col1*col1)-( SUM(col1)* SUM(col1)))

FROM dbo.test_table